You mean like this.
I couldn't get the car to start, when I pulled the cap this is what I found. This cap is less than 500 miles old. MSD 8594 dist with MSD 6AL box and blaster coil.
The cap is vented, tomorrow I will cut a window in the side of the cap so that I can see what is going on. I have a feeling that the rotor may be lower than the cap. I think I will look elsewhere for a distributor. My last distributor looked like this after two years.
MSD said it was from ozone and an unvented cap. I replaced it wth a new distributor and a vented cap and yes the new one is rusting also but the cap failed first. Search the MSD forums for rust, you might be surprised at what you find. MSD solution, drill some holes in the cap and pull the distributor, press the gear off the shaft and clean it. Apply some dielectric grease and drill a new hole for the gear retaining pin. The new hole does limit how many times you can do this.